The present invention relates to a power arithmetic apparatus for calculating power from an AC current and voltage of a target measurement system.
FIG. 1 is a block diagram showing a conventional power arithmetic apparatus.
Referring to FIG. 1, terminals T1 and T2 input a voltage V1 and a current A1 in proportion to the voltage and current of a target measurement system. The voltages V1 and A1 are converted into digital values by A/D converters 1 and 2, respectively. A CPU 3 calculates the digital values from the A/D converters 1 and 2 at a predetermined time interval.
The power arithmetic apparatus of this scheme serves as a wattmeter by calculating
P=V1xc2x7A1
and performing integration for a predetermined time, or as a watthour meter by performing infinite time integration.
However, such power arithmetic apparatus has the following problems.
(1) Since the voltage V1 and current A1 are multiplied by software, multiplication instruction processing takes a time.
(2) Since calculation is performed by software, processing is complex and time-consuming, so another processing can hardly be performed by software.
(3) Conversion using A/D converters takes a time, so the sampling frequency can hardly be increased. To increase the accuracy, the number of bits is increased, although this results in an increase in cost.
The present invention has been made in consideration of the above situation, and has as its object to provide a power arithmetic apparatus capable of performing multiplication instruction processing in a short time, avoiding complex processing to allow software to perform another processing, and preventing an increase in cost even when the sampling frequency is increased.
To achieve the above object, according to a first aspect of the present invention, there is provided a power arithmetic apparatus comprising:
means for detecting a first variation amount in a voltage in proportion to a voltage of a measuring object and a second variation amount in a voltage in proportion to a current of the measuring object; and
means for calculating power of the measuring object based on the first variation amount detected and the second variation amount detected.
According to a second aspect of the present innovation, there is provided a apparatus according to first aspect,
wherein the means for detecting the first variation amount and second variation amount comprises:
a first A/D converter for converting the voltage in proportion to the voltage of the measuring object to a first digital signal indicating the voltage in proportion to the voltage of the measuring object;
a second converter for converting the voltage in proportion to the current of the measuring object to a second digital signal indicating the voltage in proportion to the current of the measuring object;
a first counter for outputting the first variation amount based on the first digital signal; and
a second counter for outputting the second variation amount based on the second digital signal.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.